Electro-plating of metals



United States Patent ELECTRO-PLATING OF METALS James T. N. Atkinson, Dartmouth, Nova Scotia, Canada, assignor to Her Majesty the Queen in the right of galtiada as represented by the Minister of National e ence No Drawing. Application November 2, 1955 Serial No. 544,600

7 Claims. (Cl. 204--52) This invention relates to the plating of metals and more particularly it relates to a new plating process whereby satisfactory plates may be deposited upon aluminum and its alloys which hitherto have been considered refractory to normal plating methods. The present application is a continuation-in-part of my application Serial Number 468,533, filed November 12, 1954, now abandoned.

The invention will be described more particularly as applied to the plating of copper on aluminum inasmuch as copper plated aluminum has wide applications, particularly in the electrical equipment field.

Previously known method of plating copper on aluminum have been very complicated and involve the use of a series of mechanical or chemical pretreatments by means of which the protective oxide coating is removed or replaced for instance with a zinc coating. While fairly satisfactory plates can be achieved, the process is expensive and the achievement of uniform results requires a high degree of precision in control of the various steps and frequently the racking of the articles which are to be plated. Commercial quantity production on an economic basis such as can be achieved by the well-known barrel-plating process is generally considered impractical.

I have now found that strongly acid solutions have a pH of below about 4, which will dissolve the protective oxide coating but which will not appreciably attack the cathode material itself, can be used as the basis of satisfactory plating solutions. Solutions of this nature are referred to in the art as pickling solutions and for most metal which form a protective oxide film pickling solutions are known. Suitable solutions are listed in the standard corrosion handbooks amongst which may be mentioned Uhligs Handbook (John Wiley & Sons Inc., I ed. (1951)). To the pickling solution must be added a compound of the plating metal which is plates produced may be improved by the further addition to the plating solution of conventional wetting agentsand other additives of the type used to improve the characteristics of the system, as in conventional plating processes.

A suitable pickling solution for aluminum is, for instance, an aqueous solution containing about 300 grams per liter of citric acid and about 150 grams per liter of a suitable nitrate. If the nitrate chosen is cupric nitrate to provide copper as the plating metal in the fluoride. is added as a modifying agent, a very satisfactory plating solution is obtained which will produce good solderable copper plating using current densities of from about 5 to about 30 amperes per'square foot at room temperature. In solutions containing a fluoride as modifier, it is generally preferred to connect the cathode to the source of current before immersing the cathode into the bath. Gradually increasing the applied voltage is frequently helpful in improving the quality of the plate and the plating properties of the solution can be controlled and enhanced through addition to the plating solution of the usual additives, the use of which is well known in the plating art, i.-e. wetting agents, complexing agents and etc. It will be appreciated by those skilled in the art that is impossible to list exhaustively all of the plating solutions including additives or modifiers which may be used successfully. Those skilled in the plating art will, however, recognize the essential qualities necessary in a satisfactory solution and will appreciate the required modification to the picklin solution. The principal characteristics required are, of course, the availability within the plating solution of a sufiicient amount of the metal to be plated and sufiicient conductivity to permit the use of practical current densities. Satisfactory plating solutions according to the invention must contain:

(1) One or more water-soluble acids which in aqueous solution have the following properties:

(a) No reactivity or a very low rate of reactivity with the cathodic metal.

(b) Ability to form compounds or moderately stable complexes with the cation, that are soluble.

(0) At least one of the acids must, of course, dissociate sufficiently in aqueous solution to produce in the environment concerned a solution which is highly acidic, i. e. having a pH of less than about 4.

(2) A salt or other compound of the cation to be plated. If a salt is used, its anion must form or be compatible with an acid having the properties of the acids referred to in 1 and need not necessarily be the same anion as that of any of the acid compounds of thebath. The salt or other compound is simply a source When a modifier of this sort I solution and about 1 gram per liter of ammonium biof the metal to be plated and the selection of'a suitable compound will be governed by the same considerations as govern the selection of suitable sources of plating metals in conventional plating processes, i. e. the compound of the platingmetal in solution should dissociate to a suitable extent and its solubility should be such as to'provide in the solution a sufi'icient reservoir of the plating metal having regard to the current densities used.

Depending upon the composition of the cathodic metal used and the properties of the acid solution which is used, it may be found necessary or desirable to add to the solution one or more of the following:

(i) A soluble substance that promotes attack of the oxide film on the cathode; for example, a water-soluble halide, preferably a fluoride, may be used.

(ii) Suitable wetting agents amongst which may be mentioned the sodium alkyl sulphates and the sodium aryl alkyl sulphonates and similar types of compound. (iii) Suitable additives including complexing agents and chelating agents.

The process of the invention is illustrated by the following examples which it will be appreciated are exemplary only and are not intended in any sense to indicate the scope of the invention. In thefollowing examples, the criterion of adherence is a bend test.

Cupric nitr Concentrated nitric acid -cc 400 Water" to make 1 liter. Final pH about 1.0.

A current is passed through this solution using a piece of clean aluminum or aluminum alloy as the cathode at room temperature. The resulting plate coverage 1s good, but the solution is found to have relatively poor' Example 2 A plating bath was made up as follows:

Grams per liter Citric acid 300 Cupric nitrate -1 1 150 Ammonium bifluoride Final pH about 1.0.

An electric current is passed through the solution using a piece of clean aluminum or aluminum alloy as the cathode. A good plate is produced and the plating solution is found to possess reasonably good throwing power although the plate exhibits roughness and some hydrogen pitting. Immediately after plating, the plate is found to resist heating to 110 C. in an oven, to be amenable to soldering and to adhere firmly to the aluminum.

Example 3 Plating is carried out on clean aluminum or aluminum alloy using a solution similar to that in Example 2 but containing only 2 grams per liter of ammonium bifluoride. Similar results are achieved but less hydrogen pitting Was experienced.

Example 4 Plating on a piece of clean aluminum or aluminum alloy is carried out using a plating solution similar to those used in Examples 2 and 3 but using only 1 gram per liter of ammonium bifluoride. Similar results are achieved and less hydrogen pitting is experienced than noted in Example 3.

By replacing part of the citric acid with ammonium citrate in Examples 2-4 similar results are achieved at pHs ranging from 2-4.

Example 5 A plating bath was made up as follows:

Grams Boric acid 40 Nickel sulphate crystals 150 Nickel fluosilicate 50 Water to make 1 liter. Final pH was 2.7.

Example 6 The above example is repeated, but substituting nickel fluoborate for the nickel fluosilicate. The plate is somewhat more brittle than in the preceding example.

This example is repeated using added ammonia and an organic acid to keep the pH around 3. Somewhat similar results are obtained.

Examples 7 to 11 Plating baths are made up as follows:

Example Ingredients Copper Sulphate grams 150 150 Copper Nitrate. dn 150 150 Citric Acid .1 "10.-" 50 100 200 150 Sulphuric A.cid do 50 Potassium Fluori ie do 10 10 10 10 10 ater to make liters 1 12 12 12 12 Final pH 1. 4 1.1 0.95 0.8 0. 95

A current at about 15 amps. per square foot is passed through each solution using a clean aluminum cathode. The plate formed shows no signs of burning, has a fairly good appearance and gives the best adhesion in Example 7.

The present invention provides a method of plating which produces results unobtainable by previously known methods of plating and which is distinguished from previous processes by the following features:

(1) It enables the plating of aluminum and its alloys, which form protective oxide coatings, without special pretreatment other than normal cleaning of the type necessary for all conventional methods of electroplating.

(2) The process of the invention uses plating solutions that are very strongly acid, i. e. solutions having a pH of the order of 3 or 4, or less. As will be appreciated by those skilled in the art,-solutions having this order of acidity have not generally been regarded as potentially useful plating solutions for metals such as aluminum.

In connection with the plating of aluminum by the process of the present invention, suitable acids for the formation of the plating solution are: nitric acid and other strong oxidizing inorganic acids; organic acids such as citric, tartaric, oxalic, malic, lactic, other polycarboxylic and hydroxycarboxylic acids, benzoic, acetic and other non-halogenated organic acids; and certain oXy-acids of phosphorus, boron and other elements.

It is also believed that part of the function of one of the constituents of the bath may be to form a complex of moderate stability with the cation to be plated. In the first example cited, the citrate forms such a complex with copper. Complexes of moderate stability appear to coincide, according to current chemical theory, with outerorbital complexes. For a discussion of this, reference is made to an article by E. H. Lyons in the Journal of the Electrochemical Society, vol. 101 1954), p. 363-81, entitled Electronic Configuration in Electro-Deposition from Aqueous Solutions.

Modifying agents suitable for use in the plating baths described herein should increase the aggressiveness of the pickling solution toward the oxide coating on the surface of the cathodic metal and be compatible with the solution. Preferred modifying agents are those whose action can be readily controlled by varying the concentration in which they are employed, and include halides such as fluorides and particularly ammonium and potassium fluorides and bifluorides. Fluorides may usually be employed in concentrations of 0.1 or less up to one gram per liter, and sometimes higher. Too high a concentration promotes hydrogen pitting. The barrel plating technique requires a lower concentration of modifying agent than continuous techniques. 1

The expression pickling is used herein in the broad sense to mean either:

(a) Chemical treatment of a metal by immersion in an acid solution for the purpose of removing existing surfac contamination and/or replacing an existing oxide film of random properties by one of closely controlled properties with little if any removal of the'metal itself by general attack and no severe localized attack such as pitting, or

(b) Chemical treatment of a metalby immersion in an acid solution to remove all traces of corrosion product from the metal surface without removal of more than a minor amount of sound metal (as in the testing of corrosion of metals by the weight-loss method).

The terms pickling solution and pickling agen are used correspondingly to mean respectively solutions which may be used to carry out pickling and constituents which, when present in solution in suitable concentrations, produce pickling solutions.

In order to prepare aluminum or its alloys for plating, it is merely necessary to immerse them for a few minutes in a mild alkaline cleaner at 50-70" C. and then rinse them in tap water. A preliminary solvent or vapour degreasing step might be necessary for heavily soiled material.

A suitable cleaning bath would, for example, consist of an aqueous solution containing 3% each of sodium carbonate and tin trisodium phosphate, and, optionally of sodium metasilicate as inhibitor. The inhibited 1 bath would be used at the higher temperatures and for a time of about three to ten minutes, whereas the uninhibited bath would be used at a lower temperature and shorter time.

Generally speaking, alloys which have inherently good corrosion resistance, such as 1100, 5052, 6061 etc., will be able to sustain longer periods of immersion in cleaning solutions, whereas more chemically active alloys such as 7075 and 2024, will react more rapidly and only tolerate shorter cleaning treatments.

As will be realized by those who are well versed in the art, over-cleaning can result in the formation of an objectionable smut which can be removed by the use of an acid dip appropriate to the alloy involved. It may then be necessary to return the aluminum alloy to the cleaning bath for a shorter period before continuing with the plating step.

In the appended claims the word cathodic is used to indicate the metal which constitutes the cathode which is to be plated, and the term plating to indicate the metal which is to be deposited on the said cathodic metal.

What I claim is:

1. A method of electro-plating copper directly on the surface of a cathodic metal member selected from the group consisting of aluminum and aluminum-base alloys which comprises immersing an anode and said cathodic metal member into an aqueous plating bath having a pH not over about 4.0, said aqueous bath containing about a 100-150 grams per liter of a copper salt selected from the group consisting of copper sulfate and copper nitrate, about 100-300 grams per liter of a water-soluble acid selected from the group consisting of citric acid and a. -50 mixture of citric acid and sulfuric acid, and about 1-10 grams per liter of a water-soluble fluoride, and passing an electric current through said plating bath between said cathodic metal member and said anode.

2. The method set forth in claim 1 wherein the watersoluble fluoride is potassium fluoride.

3. The method set forth in claim 1 wherein the watersoluble fluoride is ammonium bifluoride.

4. The method set forth in claim 1 wherein the cathodic metal member is connected to the source of electric current before being immersed in the plating bath solution.

5. The method set forth in claim 1 wherein the current density is from about 5 to about 30 amperes per square foot and the method is carried out substantially at room temperature.

6. The method set forth in claim 1 wherein the plating bath solution comprises about 150 grams per liter of citric acid, about 10 grams per liter of potassium fluoride and about 150 grams per liter of copper sulfate.

7. The method set forth in claim 1 wherein the plating bath solution comprises about 300 grams per liter of citric acid, about 1 gram per liter of ammonium bifluoride and about -150 grams per liter of copper nitrate.

References Cited in the file of this patent UNITED STATES PATENTS 946,903 Kern Ian. 18, 1910 2,437,865 Stareck Mar. 16, 1948 2,728,720 DeLong Dec. 27, 1955 

1. A METHOD OF ELECTRO-PLATING COPPER DIRECTLY ON THE SURFACE OF A CATHODE METAL MEMBER SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM-BASE ALLOYS WHICH COMPRISES IMMERSING AND ANODE AND SAID CATHODIC METAL MEMBER INTO AN AQUEOUS PLATING BATH HAVING A PH NOT OVER ABOUT 4.0 SAID AQUEOUS BATH CONTAINING ABOUT 100-150 GRAMS PER LITER OF A COPPE R SALT SELECTED FROM THE GROUP CONSISTING A COPPER SULFATE AND COPPER NITRATE ABOUT 100-300 GRAMS PER LITER OF A WATER SOLUBLE ACID SELECTED FROM THE GROUP CONSISTING OF CITRIC ACID AND A 50-50 MIXTURE OF CITRIC ACID AND SULFURIC ACID, AND ABOUT 1-10 GRAMS PER LITER OF A WATER-SOLUBLE FLUORIDE, AND PASSING AN ELECTRIC CURRENT THROUGH SAID PLATING BATH BETWEEN SAID CATHODIC METAL MEMBER AND SAID ANODE. 